In view of the recent diversification of can-manufacturing methods for food containers, electric resistance seam welding methods such as a copper wire resistance seam welding method, etc., have been developed considerably as can-manufacturing methods.
As shown in FIG. 1, the copper wire resistance seam welding method is a method in which the longitudinal edge portions of a can shell sheet (1) preliminarily formed into a cylindrical form are passed, while being overlapped each other to a predetermined width, through a copper wire (2) wrapped round a pair of roller electrodes (3) which are arranged one upon another and at this time the application of pressure and the passage of current through the overlapped portions of the can shell sheet (1) are effected by the top and bottom roller electrodes (3) through the copper wire (2) thereby effecting the welding continuously by the heat generated by the electric resistance of the overlapped portions.
Here, as shown in FIG. 2, a guide bar (4) having a cross-sectional shape lime Z is used and the longitudinal edge portions of the can shell (1) are inserted into the guide slots of the guide bar 4 thereby overlapping them. This overlapping width is determined by the Z-shaped guide bar (4) for overlapping the longitudinal edge portions and can-shell bore determining rolls arranged around the top and bottom roller electrodes (3).
The quality of the weld zone is evaluated in terms of the bonding strength, hermetic sealing properties and external appearance properties of the seam weld zone. The bonding strength and the hermetic sealing properties are problems which have bearing on the leakage of the contents and to satisfy these characteristics it is essential that the heat generated by the electric resistance at the seam interface is greater than a certain limit value. For this reason, a minimum value of the welding current is determined. Also, as regards the external appearance properties, it is essential that there is no flash. While the seam weld is usually covered with a protective coating after the welding, the presence of any flash impedes the application of the coating or it produces an exposed portion which is not covered with the coating and moreover the use of such can shell has the danger of not only causing the contents to deteriorate as a result of this reaction with the flash but also causing the flash to fall off the can shell and enter into the contents. The flash is formed by the molten metal scattered from the seam weld and sticked to therearound. In order to prevent the presence of any flash, it is essential that the heat generated by the electric resistance at the seam interface is less than a certain limit value. Thus, a maximum value of the welding current is determined.
If the minimum and maximum values of the welding current which are determined in consideration of the bonding strength, hermetic sealing properties and external appearance properties for evaluating the quality of the seam weld are respectively defined as a lower limit current value and an upper limit current value, the value obtained by subtracting the lower limit current value from the upper limit current value represents a range of positive values for a range of proper current values for the welding and the quality of weldability of a material for container is determined by the extent of its range of proper current values.
The steel materials for containers used with this welding method are required that they are not only excellent in these performances required for sheet steels for containers such as corrosion resistance, workability and coating properties but also excellent in weldability.
While the steel materials for containers have heretofore consisted mainly of the tin plates (the weights of tin coating are 2.8 g/m.sup.2 or over) for soldered cans, new materials (steel materials) have recently been developed. Where these materials are used as objects of the electric resistance seam welding method, the tin plates used for soldered cans and a part of the lightly-coated or washed tin plates have large ranges of proper current values and they are very excellent in weldability. However, the remainder of the lightly-coated tin plates and the tin-free steels are small in the range of proper welding current values and low in weldability as compared with the tin plates for soldered cans.
The tin-free steel tends to produce flashes during the welding and a reduction in the welding current value for preventing the formation of flashes tends to make the seam strength and the adhesion properties unsatisfactory.
Therefore, it has been the practice in the past to perform the electric resistance seam welding after the surface coating on the blank edge portions forming the overlaping portions have been preliminarily removed by mechanical means such as a wire brush thus substantially exposing the steel surface wholly. However, the use of such mechanical means increases the operating steps by one step and moreover the scraped fine particles are not removed completely from the surface of the steel sheet and stick again thus giving rise to various inconveniences during the subsequent operations. Also, there are disadvantages that the exposed steel surface subsequent to the removal of the surface coating has no corrosion resistance, that its external appearance is deteriorated and so on. As a result, the tin-free steel has not been used for the manufacture of cans for canned food as yet.
It is an object of the present invention to provide an electric resistance seam welding method which overcomes the foregoing deficiencies and by which materials for food containers which are difficult to be subjected to electric resistance welding such as the tin-free steel can be easily welded without requiring any special preliminary treatment.